Memory has many uses in modern integrated circuitry. One known type of memory cell is a static memory cell, such as may be utilized in static random-access memory (SRAM). A static memory cell is characterized by operation in one of two mutually-exclusive and self-maintaining operating states. Each operating state defines one of the two possible binary bit values, zero or one. A static memory cell typically has an output which reflects the operating state of the memory cell. Such an output produces a “high” voltage to indicate a “set” operating state. The memory cell output produces a “low” voltage to indicate a “reset” operating state. A low or reset output voltage usually represents a binary value of zero, while a high or set output voltage represents a binary value of one.
A static memory cell is said to be bistable because it has two stable or self-maintaining operating states, corresponding to two different output voltages. Without external stimuli, a static memory cell will operate continuously in a single one of its two operating states. It has internal feedback to maintain a stable output voltage, corresponding to the operating state of the memory cell, as long as the memory cell receives power.
The operation of a static memory cell is in contrast to other types of memory cells, such as dynamic memory cells, which do not have stable operating states. A dynamic memory cell can be programmed to store a voltage which represents one of two binary values, but requires periodic reprogramming or “refreshing” to maintain this voltage for more than very short time periods.
A dynamic memory cell has no internal feedback to maintain a stable output voltage. Without refreshing, the output of a dynamic memory cell will drift toward intermediate or indeterminate voltages, resulting in loss of data. Dynamic memory cells are used in spite of this limitation because of the significantly greater packing densities which can be attained. For instance, a dynamic memory cell can be fabricated with a single MOSFET transistor, while static memory cells comprise more than one transistor. Because of the significantly different architectural arrangements and functional requirements of static and dynamic memory cells and circuits, static memory design has developed along generally different paths than has the design of dynamic memories.
In semiconductor processing, there is a continuing desire to make circuits denser, and to place components closer together to reduce the size of circuits. It would be desirable to develop improved architectures for static memory which are suitable for utilization in highly integrated applications.